Reducing memory usage in noncoherent signal processing

ABSTRACT

A method for detecting a positioning signal includes (a) correlating a segment of a received positioning signal with a reference signal of a selected code phase and frequency to obtain a correlation value, (b) if the correlation value is less than a predetermined minimum, assigning the correlation value to the predetermined minimum, and (c) accumulating the correlation value in a sum of correlation values obtained using other segments of the received positioning signal. In addition, the correlation value may be reduced by a predetermined value, which is preferably an expected mean value for a noise component in the segment of the received positioning signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to signal detection techniques. Inparticular, the present invention relates to reducing memory usage insystems and methods that use non-coherent accumulation in detectingsignals.

[0003] 2. Discussion of the Related Art

[0004] A receiver of a navigation system detects positioning signalsfrom the Global Positioning System (GPS) to estimate its location. Onecommon detection method uses non-coherent accumulation, which is atechnique for searching for the code phase and frequency of a receivedsignal involving accumulating either the modulus, or the squaredmodulus, of complex correlations of a reference sginal and the receivedsignal. Depending upon the relative amount of signal and noise presentin the received signal, the correlation values will have differentdistributions. Typically, an accumulated sum is stored in memory foreach trial combination of code-phase and frequency. Signal detectionoccurs when the statistical properties of an accumulated sum indicatethat a signal is almost certainly present. Memory is a significant costin such a navigation system, as a large number of such accumulated sumsare kept in memory to achieve high resolution in detection.

[0005] A number of U.S. patent applications disclose applicable signalprocessing techniques relevant to a navigation system of the presentapplication:

[0006] 1. Signal Acquisition using Data Bit Information (Ser. No.09/888,228, “the '228 patent application”) filed Jun. 22, 2001;

[0007] 2. Synthesizing Coherent Correlation Sums at One or MultipleCarrier

[0008] 3. Frequencies Using Correlation Sums Calculated at a Coarse Setof Frequencies (Ser. No. 09/888,227, “the '227 patent application”),filed Jun. 22, 2001;

[0009] 4. Extracting Fine-Tuned Estimates from Correlation FunctionsEvaluated at Limited Number of Values (Ser. No. 09/888,338, “the '338patent application”), filed Jun. 22, 2001;

[0010] 5. Determining the Spatio-Temporal and Kinematic Parameters of aSignal Receiver and its Clock by Information Fusion (Ser. No.09/888,229, “the '229 patent application”), filed Jun. 22, 2001;

[0011] 6. Determining Location Information Using Sampled Data ContainingLocation Determining Signals And Noise (Ser. No. 09/888,337, “the '337patent application”). filed Jun. 22, 2001;

[0012] 7. Method for optimal search scheduling in satellite acquisition(Ser. No. 10/126,853, “the '853 patent application”), filed on Apr. 19,2002;

[0013] 8. System and method to estimate the location of a receiver in amulti path environment (Ser. No. 10/237,556, “the '556 patentapplication”), filed on Sep. 6, 2002;

[0014] 9. System and method to estimate the location of a receiver (Ser.No. 10/237,557, “the '557 patent application”), filed on Sep. 6, 2002;and

[0015] 10. Multifunction device with positioning system and sharedprocessor (Ser. No. 10/286,360, “the '360 patent application”), filed onNov. 1, 2002.

[0016] The above copending patent applications, which are each assignedto SirF Technology, Inc., are hereby incorporated by reference in theirentirety.

SUMMARY OF THE INVENTION

[0017] In a signal detection application, the present invention providesa method for reducing the memory requirements for storing accumulatedsums in memory. According to one embodiment of the present invention,the dynamic range of the accumulated sum is quantized to a specifiedfinite number of values. To efficiently use memory, the dynamic range isallocated to the signal portion, rather than the noise portion, of theaccumulated sum.

[0018] Thus, according to one embodiment of the present invention, amethod for detecting a positioning signal includes (a) correlating asegment of a received positioning signal with a reference signal of aselected combination of code phase and frequency to obtain a correlationvalue, (b) if the correlation value thus obtained is less than apredetermined minimum, assigning the correlation value to thepredetermined minimum, and (c) accumulating the correlation value in asum of correlation values obtained using other segments of the receivedpositioning signal. In addition, the correlation value may be reduced bya predetermined value. In one embodiment, the correlated value isreduced by an expected mean value for a noise component in the segmentof the received positioning signal.

[0019] In one embodiment, the correlation value is quantized, with thepredetermined minimum value corresponding to the least quantizedcorrelation value.

[0020] In one embodiment, the sum of correlated values is compared to apredetermined threshold value corresponding to the greatest quantizedvalue. When the sum of the correlated values exceeds the predeterminedthreshold value, the accumulating of the sum of correlated values is notfurther carried out for additional segments of the received positioningsignal.

[0021] The methods of the present invention provides a memory-efficientnon-coherent accumulation.

[0022] The present invention is better understood upon consideration ofthe detailed description below and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 illustrates a process of non-coherent accumulation.

[0024]FIG. 2 illustrates a process providing non-coherent accumulationwith reduced memory usage, in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention provides a system and a method thatimproves memory usage efficiency of signal detection using non-coherentaccumulation over systems and methods of the prior art. In aconventional GPS application, for example, a received positioning signalof a certain duration is divided into a large number of segments, say N,and correlated with a locally generated replica of the expected GPSsignal (i.e., the code expected to be received from a GPS satellite,modulated by the expected carrier signal). If the sequence of the Ncomplex correlation values thus computed is denoted by x_(i), anon-coherent accumulation of the modulus of x_(i) can be defined as:$\sum\limits_{i = 1}^{N}{x_{i}}$

[0026] Alternatively, the accumulated value can be a sum of the squaredmodulus of X_(i) (i.e.,$\left( {{i.e.},{\sum\limits_{i = 1}^{N}{x_{i}}^{2}}} \right),$

[0027] or some other power of x_(i), to optimize memory usage, detectionsensitivity or other performance parameters.

[0028] Let σ² denote the noise power in each of the real and imaginaryparts of x_(i) (i.e., the total noise power is 2σ²), and let M denotethe probability distribution of |x_(i)|, which are assumed identicallydistributed. If the received signal contains only Gaussian white noise,then the probability density function of ƒ(M) follows a Rayleighdistribution:${f(M)} = {\frac{M}{\sigma^{2}}^{\frac{M^{2}}{2\quad \sigma^{2}}}}$

[0029] with a mean of M given by $\mu = {\sqrt{\frac{\pi}{2}}\sigma}$

[0030] and a standard deviation of M given by$\sigma_{M} = {\sqrt{2 - \frac{\pi}{2}}{\sigma.}}$

[0031] However, if the received signal contains a GPS signal, then let Sdenote the carrier to noise ratio $\frac{C}{N_{0}}$

[0032] for the received signal, and the probability density functionƒ^(SIG) (M) follows a Ricean distribution:${f^{{SIG}\quad \sigma^{2}}(M)} = {\frac{M}{\sigma^{2}}^{\frac{M^{2} + a^{2}}{2\quad \sigma^{2}}}{I_{0}\left( \frac{a\quad M}{\sigma^{2}} \right)}}$

[0033] with a mean of M given by$\mu_{M} = {\frac{\pi}{2}{\sigma \left( {{\left( {1 + r} \right){I_{0}\left( \frac{r}{2} \right)}} + {r\quad {I_{1}\left( \frac{r}{2} \right)}}} \right)}\quad ^{- \frac{r}{2}}}$

[0034] where ${r = {T\left( 10^{\frac{S}{10}} \right)}},$

[0035] T being the coherent block size in seconds, and I₀ being thezero-th order modified Bessel function of the first kind.

[0036]FIG. 1 outlines generally the steps in a process providingnon-coherent accumulation. As shown in FIG. 1, at step 101, the modulusof a correlation value over a segment of the received signal iscalculated. At step 102, which is an optional step, the modulus of thecorrelation value is quantized. At step 103, the modulus is added to asum accumulating the modulus of correlation values of all the segmentsof received signal. The quantization performed at step 102 allows theaccumulating sum to be represented by a lower precision than theprecision of the computation in step 101. In determining the appropriatequantization, the memory costs is balanced against the precision lossdue to quantization. In one embodiment, a quantization of$\frac{\sigma}{2}$

[0037] is applied. In the $\frac{\sigma}{2}$

[0038] quantization, the modulus of the correlation value is divided by$\frac{\sigma}{2}$

[0039] and the result is then rounded to the nearest integer.Alternatively, instead of rounding, a floor or ceiling function can beapplied.

[0040] Non-coherent accumulation achieves two functions. First, signaldetection is achieved by comparing the accumulated sum to the expectedprobability distribution of a received signal that contains only noise.When the accumulated sum differs from that expected probabilitydistribution in a statistically significant way, a signal is deemeddetected. Second, upon detection, the frequency and code phase estimatescan be further refined by interpolating accumulation values near thepeak accumulation. In other words, the code phases and frequencies nearthe peak accumulation can be used to further improve the estimates ofcode phase and frequency.

[0041]FIG. 2 illustrates in further detail a process providingnon-coherent accumulation with reduced memory usage, in accordance withone embodiment of the present invention. As shown in FIG. 2, at step201, the modulus of a correlation value over a segment of the receivedsignal is computed. At step 202, the modulus calculated at step 201 isreduced by a predetermined value. In one embodiment, the modulus isreduced by the expected value of the noise-only modulus$\left( {{i.e.},{\sigma_{M} = {\sqrt{2 - \frac{\pi}{2}}{\sigma.}}}} \right).$

[0042] At step 203, the reduced modulus is quantized. (Althoughreduction of the modulus by σ_(M) is described above as being carriedout at step 202 before quantization step 203 at a higher precision,reduction of the modulus can also occur after quantization, using aquantized value of σ_(M))

[0043] At step 204, the quantized reduced modulus is accumulated in anaccumulated sum. If the accumulated sum is less than a specified minimumvalue L, the accumulation sum is set to L (the “saturation” step). Thesaturation step sacrifices the ability to distinguish among lowaccumulations. However, for an appropriately chosen minimum value L, asthe low accumulations are highly unlikely to contain a detectablepositioning signal, the impact of the saturation step on signaldetection is negligible. In one embodiment, where non-coherentaccumulations are stored using 8 bits and the quantization is achievedby dividing by $\frac{\sigma}{2},$

[0044] the minimum value L is set to −8. L may be set to a value evenless than −8 to further reduce the impact due to the saturating step onthe distribution of the accumulated sum. L may also be set to a valuehigher than −8 to provide a larger useful dynamic range.

[0045] In step 205, the accumulation sum is compared against a highthreshold value H. If the accumulated sum exceeds the high thresholdvalue H, accumulations for the satellite of the current search arehalted, so that all accumulations will have processed the same amount ofreceived signal. By halting accumulations, the accumulations near thepeak are not saturated, and methods such as those described in the '338application can be applied to further refine estimates of code phase andfrequency. In the embodiment described above (i.e., 8-bit accumulations,$\frac{\sigma}{2}$

[0046] quantization, and minimum value L=−8), a high threshold value His set to the maximum quantized value, given by 255−8=247.Alternatively, high threshold value H may be a value less than themaximum value to avoid flattening the shape of the ambiguity peak whenthe last term in the accumulated sums is added. If the high thresholdvalue H is not met, then the signal is detected by comparing theaccumulated sums to a lower threshold determined according to theapplications incorporated above.

[0047] According to the embodiment of the present invention describedabove, memory usage in the non-coherent accumulation is greatly reduced,while still allowing signal detection and estimation refinement.

[0048] The above detailed description is provided to illustrate thevarious embodiments of the present invention and is not intended to belimiting. Numerous modifications and variations within the scope of thepresent invention are possible. The present invention is set forth inthe claims below.

We claim:
 1. A method for detecting a positioning signal, comprising:correlating a segment of a received positioning signal with a referencesignal of a selected code phase and frequency to obtain a correlationvalue; if the correlation value is less than a predetermined minimum,assigning the correlation value to the predetermined minimum; andaccumulating the correlation value in a sum of correlation valuesobtained using other segments of the received positioning signal.
 2. Amethod as in claim 1, further comprising reducing the correlation valueby a predetermined value.
 3. A method as in claim 2, wherein thecorrelation value is reduced by an expected mean value for a noisecomponent in the segment of the received positioning signal.
 4. A methodas in claim 1, further comprising quantizing the correlation value.
 5. Amethod as in claim 4, wherein the predetermined minimum value is theleast quantized correlation value.
 6. A method as in claim 1, furthercomprising comparing the sum of correlated values to a predeterminedthreshold value.
 7. A method as in claim 6, wherein the accumulating isnot further carried out for additional segments of the receivedpositioning signal when the sum of correlated values exceeds thepredetermined value.
 8. A method as in claim 6, further comprisingquantizing the correlation value, and wherein the predeterminedthreshold value is the greatest quantized value.